It is known to use a radiating coaxial cable transmission line, also commonly referred to as a “leaky wave coaxial cable” or simply a “leaky cable,” as an antenna. Long line leaky cable antennas are useful for communication applications where a point source antenna is inadequate—e.g., in tunnels, mines, along roadways, or the like. Standard coaxial cables are modified to have slots, loose braids, or helical shields designed so that communication signals leak out over long distances, e.g., several miles or kilometers. Fundamentals of known manner radiating coaxial cable technology are described in U.S. Pat. No. 3,417,400 (Black, December 1968), incorporated herein by reference. Basics of apertured radiating coaxial cables are described in U.S. Pat. No. 4,339,733 (Jul. 13, 1982) to Smith for a Radiating Cable, incorporated herein by reference. Radiating Cables Having Spaced Radiating Sleeves are described by Hildebrand, et al. in U.S. Pat. No. 4,129,841, incorporated herein by reference.
The electromagnetic radiation (EM) modes that a leaky cable antenna supports are shown in FIG. 1 (Prior Art). The term “electromagnetic waveband” is used herein for any audio, visual, microwave, computer broadband, and the like communications signals extant in the state-of-the art. A leaky cable is generally a two-wire transmission line that may support two modes: a bifilar mode and a monofilar mode. Referring now to FIG. 1 section (a), a leaky cable 101a is constructed much like an ordinary solid coaxial cable having a center conductor 102, but modified with a spiral, helical, outer conductor 106. The outer conductor 106 is wound with a substantially constant pitch as it traverses around the outer surface of and down the longitudinal axis of an inner dielectric insulator 104 surrounding the center conductor 102. The cable 101a similarly includes an outer insulator 108. EM fields (represented by radiating arrows) with respect to cable 101a are largely concentrated between the two conductors, known as the bifilar mode. That is the EM fields generally non-radiative with respect to the local environment. Looking now to FIG. 1 section (b), a cable 101b in the monofilar mode is similar to that of a single-wire transmission line with a dielectric coating. It is radiative at discontinuities in the construct. The EM fields with respect to cable 101b are largely concentrated in the air region around the cable. There is no return conductor in this device; the return path may be considered to be a notional ground plane that is an infinite distance from the cable 101b. It is known that if two counter-wound helical outer conductors have exactly the same pitch, such a cable performs similarly to ordinary single helical coaxial cable. See e.g., Electromagnetic Theory of the Loosely Braided Coaxial Cable: Part I, Wait, J. R., IEEE Transactions on Microwave Theory and Techniques, Vol. MIT-24, No. 9, September 1976, incorporated herein by reference.
On a perfectly uniform cable, neither of these two modes radiate a substantial EM field since their phase velocity is slower than the speed of light. The phase velocity of the bifilar mode is governed by the inner dielectric insulator. However, the phase velocity of the monofilar mode is just slightly slower than the speed of light, which means that it is only loosely bound to the cable, and extends a significant distance into free space. In this mode EM radiation is easily scattered by discontinuities or bends in the cable. Thus, traditional helical wound leaky cable antennas work over long distances because of the constant flow of energy between these two weakly coupled modes, with the monofilar mode gradually leaking power into the surrounding space. As a result, conventional leaky cable antennas typically require hundreds of wavelengths or more to radiate efficiently, making them suitable for use as multiband antennas inside tunnels, mines, along roadways or the like.
Such leaky cable antennas provide additional environmental robustness compared to a single point radiator type (such as a bow-tie antenna) in that if a portion of the cable is shorted out, such as by moisture or nearby conductive surfaces, the energy simply continues down the cable and radiates from the next available radiator region.
There have been developed specifications for military antenna systems—such as that described by R. C. Adams, R. S. Abramo, J. L. Parra, J. F. Moore, “COMWIN Antenna System Fiscal Year 200 Report,” SPAWAR, San Diego, Calif., Technical Report 1836, September 2000—where leaky cable antennas have been employed but are usually designed for broadband application. Use in proximity to military personnel has raised series radiation hazard issues. Prototypes described therein also exhibit signal distribution patterns sometimes having one or more null regions, thus exhibiting a relatively lesser operational efficiency. Another problem is that military specifications define specific signal polarizations for particular applications. Thus, field antenna systems must perform accordingly depending upon the transmission source protocols in use during current operations.
Another adaptation for using a leaky cable antenna is for covert operations—such as for investigative or military applications—where it is desirable to mask the visual signature of the user of antenna-related communication devices. It has been found that a wearable antenna is advantageous. However, for such applications, problems related to signal transmission—e.g., bandwidth and directionality capabilities, field effects due to proximity of a human body, and the like—and to wearability—e.g., disguisability, weight and flexibility, robustness, radiation hazards protection, and the like—must be accounted for in the design.
There is a need for leaky cable antenna devices and methods which address the foregoing issues.